Does the background vary?

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Yes, the same components are found in the HRI background as in the PSPC
background, but at different rates. All except the particle background
originate externally to the satellite as X-rays, so the count rates are
only different by the relative effective areas of the two instruments. The
particle background is relatively higher for the HRI than for the PSPC and
is in general the dominant background component providing count rates
comparable to the cosmic background.